Heretofore, a multilayer printed circuit boards, have been made up of organic high molecular compounds as glass epoxy resins since these materials have a low dielectric constant of about 4 and are simple to manufacture. It is, however, impossible to directly mount integrated circuit (IC) chips on these multilayer circuit boards because of their poor thermal resistance. In addition, it is impossible with such a material to produce multilayer printed circuit boards comprising 20 to 30 insulated internal layers.
To this end, multilayer printed circuit boards employing an alumina ceramic, as a material for insulators, have been developed and put into practical use. However, the use of alumina ceramics provides new problems because of its large dielectric constant of 9 to 10. With speeding up of operations of such electronic devices as super computers, there is an increasing tendency to use clock pulses with a high frequency of 100 to 300 MHz. At such high frequencies, the greater the dielectric constant of the insulator for multilayer boards, the greater is the delay of propagation of signals in the area where the conductors with a long length are provided. In addition, the use of a large dielectric constant insulator causes an increase of electrostatic capacitance between neighboring wiring conductors, which in turn results in attenuation of signals as well as a decrease of circuit impedance.
In order to solve these problems, there have been proposed a few multilayer printed circuit boards with new constructions. For example, Japanese patent published No. 39077/1981 discloses a multilayer printed circuit board comprising an alumina substrate and conductor layers separated by an insulating layer; each insulating layer being arranged between conductor layers and composed of a low dielectric constant material consisting essentially of 50 to 64% of SiO.sub.2, 18 to 25% of B.sub.2 O.sub.3, 2 to 3% of Al.sub.2 O.sub.3, 1 to 2% of Na.sub.2 O, 1 to 3% of K.sub.2 O, 6 to 9% of PbO, 0.25 to 0.5% of CaO and 0.25 to 0.5% of MnO.sub.2.
Japanese patent laid-open No. 15098/1981 discloses a multilayer printed circuit board comprising a ceramic substrate on which several metallized layers and thin ceramic layers are alternately stacked on one another. A cavity layer parallel to the metallized layer is provided in the thin ceramic layer located between adjacent metallized layers.
However, there is an increasing demand for development of multilayer printed circuit boards with a lower dielectric constant that can be easily manufactured.